Restore search engine indexing after launch

# Description/Context
All environments, including production, currently include a `<meta name="robots" content="noindex">` tag, which prevents search engine indexing. (https://github.com/mitodl/mit-open/blob/main/open_discussions/templatetags/noindex_meta.py)

When we are ready to launch, we should include this tag only on non-production environments.
